Kinetic theory of the electron strahl in the solar wind

Abstract

We develop a kinetic theory for {the electron strahl, a beam of energetic electrons which propagate} from the sun along the Parker-spiral-shaped magnetic field lines. By assuming a Maxwellian electron distribution function in the near-sun region where the plasma is collisional, we derive the strahl distribution function at larger heliospheric distances. We consider the two most important mechanisms that broaden the strahl: Coulomb collisions and interactions with oblique ambient whistler turbulence (anomalous diffusion). We propose that the energy regimes where these mechanisms are important are separated by an approximate threshold, ${\mathcal{E}}_c$; for the electron kinetic energies $\mathcal{E}<{\mathcal{E}}_c$ the strahl width is mostly governed by Coulomb collisions, while for $\mathcal{E}>{\mathcal{E}}_c$ by interactions with the whistlers. The Coulomb broadening decreases as the electron energy increases; the whistler-dominated broadening, on the contrary, increases with energy and it can lead to efficient isotropization of energetic electrons and to formation of the electron halo. The threshold energy ${\mathcal{E}}_c$ is relatively high in the regions closer to the sun, and it gradually decreases with the distance, implying that the anomalous diffusion becomes progressively more important at large heliospheric distances. At $1$ AU, we estimate the energy threshold to be about ${\mathcal{E}}_c\sim 200\mathrm{eV}$.

arXiv commentsubmitted to MNRAS

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Cited by in corpus (16)

• Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum
• Electron heat flux in the near-Sun environment
• Particle-In-Cell simulation of whistler heat flux instabilities in the solar wind: heat flux regulation and electron halo formation
• On the temperature of the solar wind
• Whistler wave occurrence and the interaction with strahl electrons during the first encounter of Parker Solar Probe
• Precision Electron Measurements in the Solar Wind at 1 au from NASA's Wind Spacecraft
• Parker Solar Probe evidence for scattering of electrons in the young solar wind by narrowband whistler-mode waves
• Scattering of energetic electrons by heat-flux-driven whistlers in flares
• Whistler instability driven by the sunward electron deficit in the solar wind
• Modeling interactions of narrowband large amplitude whistler-mode waves with electrons in the solar wind inside ~.3 AU and at 1 AU using a particle tracing code
• Stability of superthermal strahl electrons in the solar wind
• Particle energization in colliding subcritical collisionless shocks investigated in the laboratory
• Quantifying the diffusion of suprathermal electrons by whistler waves between 0.2 and 1 AU with Solar Orbiter and Parker Solar Probe
• The Kinetic Expansion of Solar-Wind Electrons: Transport Theory and Predictions for the very Inner Heliosphere
• The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons
• Ambipolar electric field and potential in the solar wind estimated from electron velocity distribution functions